My invention relates to a tone frequency detecting circuit, and particularly to a tone frequency detecting circuit for detecting the frequency of an audio tone transmitted in a radio communication system.
In some radio communication systems, it may be desirable or necessary that some transmission on a particular radio carrier frequency be heard only by a selected receiver or receivers, and that some other transmissions on the same particular radio carrier frequency be heard only by a different selected receiver or receivers. Such a system is sometimes referred to as a channel guard system. In such systems, an audio tone of selected frequency is transmitted along with but outside the range of the information or voice frequencies. The particular frequency of the audio tone is used to render selected radio receivers operative or activated so that only those receivers sensitive to the frequency of the audio tone reproduce the information transmitted with the tone, and so that all other radio receivers remain inoperative or inactivated. Thus, more effective use can be made of a single radio carrier frequency, and some privacy can still be provided.
Radio industry standards have established 26 different audio tone frequencies between 71.9 and 203.5 hertz for use in such systems. This means that the average frequency separation of adjacent tones is approximately 5.3 hertz. Hence, the frequency detecting circuit should be very selective or narrow in its response. If the frequency detecting circuits use physical inductors and capacitors, such circuits are very large and heavy.
Accordingly, a primary object of my invention is to provide a new and improved tone frequency detecting circuit, particularly for use in tone actuated radio receivers.
Another object of my invention is to provide a new and improved tone frequency detecting circuit that can accurately detect a selected audio tone having a relatively low frequency and having relatively little frequency separation between adjacent audio tones.
Another object of my invention is to provide a new and improved tone frequency detecting circuit that can detect a relatively low frequency audio tone but that can be in the form of relatively small and light microelectronic circuits.